Liferay follows strict naming conventions for themes and their resources, including folder and file names. A theme's images will be located in the images folder within the theme's root directory, e.g: eggplant-theme\images. Within the images folder there are 22 sub-folders with names category names like dock, navigation, and portlet. It is important to know the proper locations for theme images and then also what to name them, and the creation of new image folders is strongly discouraged.

In previous versions, Liferay has had the ability to create virtual themes-within-themes, allowing theme creators to express different color schemes that visually retain the same look and feel of original. In Liferay 4.3 and above, this concept has been expanded to allow greater flexibility to child themes / sub-themes development.

images/color_schemes/ is a virtual images differences folder for a theme with multiple color schemes, although it is no longer limited to color schemes. Since images cannot inherently have dynamic color, hue, or other visual transformations applied to them, some images may need to be replaced to express a difference in a differing color scheme / sub-theme.

images/color_schemes/theme-name/ acts as a virtual root for these necessary image replacements. More precisely, inside the color_schemes/ folder exists a series of sub-folders for each scheme (eg. blue/, green/ orange/). These sub-folders are the equivalent of the theme's root images folder ( eg. green versions of images within the images/ folder would be placed into the images/color_schemes/green/ folder. )

There's the notion of a "Content Delivery Network". I've not used this, but you might want to look for this in the documentation. Look at the image source for this page: the page is served from http://www.liferay.com/ while images are from http://cdn.www.liferay.com/